The Oscar-winning Godfather star revealed he wanted to pay tribute to his dad with Remembering The Artist: Robert De Niro Sr.

"I felt I had to. I felt obligated. It was my responsibility to make a documentary about him. I was always planning on doing it, but never did," he told Out magazine.

"Then Jane Rosenthal, my partner at Tribeca [Enterprises], said, 'We should start doing that now.' It was not intended to be on HBO. It was just something I wanted to do."

Robert De Niro Sr, who died in May 1993 aged 71, was an artist, who was also gay. The documentary, which will be shown on HBO on June 9, will showcase his artwork and shed light on his legacy, as well as his conflicted emotions about his sexuality.

"Yeah, he probably was [conflicted], being from that generation, especially from a small town up-state," Robert explained.

"I was not aware, much, of it. I wish we had spoken about it much more. My mother didn't want to talk about things in general, and you're not interested when you're a certain age. Again, for my kids, I want them to stop and take a moment and realise that you sometimes have to do things now instead of later, because later may be 20 years from now - and that's too late."

Robert continues to preserve his father's final home and art studio in SoHo, New York.

The 70-year-old said: "It was the only way to keep his being, his existence alive. To me, he was always a great artist."